Engines, such as aircraft engines, can generate significant noise. Such noise may be undesirable in populated areas and in other environments in which noise is desirably controlled. As such, acoustical liners for inlets, fan cases, fan nozzles, and other engine installation structures have been developed to reduce the amount of noise emanating from an engine. The acoustical liners generally are disposed within the nacelle of the aircraft engine.
In a turbofan engine, for example, the portion of the inlet portion of the nacelle forward of the fan includes inner and outer barrels separated by an air-filled space. In order to reduce the noise emanating from the engine, the inner barrel can incorporate an acoustical liner. An acoustical liner generally includes a cellular or honeycomb core positioned between a face sheet which faces the air flowing toward the fan and a back skin which faces the outer barrel. The face sheet may be perforated such that some of the acoustic air flow to or through the fan enters the honeycomb core through the perforations in the face sheet. As a result of the interaction of a portion of the air flowing to or through the fan with the honeycomb core, the noise emanating from the engine is reduced. In order to further reduce the resulting noise, a septum may be disposed within the honeycomb core. By controlling the size and number of the perforations as well as the volume of air within the honeycomb cells and the properties of the septum, the performance of the acoustical liner can be tuned to reduce noise in a particular frequency range. In this regard, the perforations through the face sheet provide acoustical inertia, while the volume of air contained in the honeycomb cells provide acoustical compliance, thereby providing a dynamic system with a limited number of acoustical degrees of freedom.
In addition to the noise generated by an engine, another issue associated with turbofan engines is fan blade flutter which may reduce the useful lifetime of the fan blades and, in some situations, may cause the fan blades to fail. In instances in which the fan blades are anticipated to flutter, the fan blades are generally scheduled to be inspected on a more frequent basis, and the lifetime of the fan blades is typically limited relative to fan blades that are not anticipated to flutter. In an effort to eliminate or reduce fan blade flutter, the fan nozzle geometry, that is, the converging/diverging characteristics of the nozzle, may be changed. However, such changes are constrained by the thrust requirements for the engine, and may disadvantageously add to the weight of the aircraft or undermine the structural integrity of the nacelle. Because fan blade flutter involves interaction between vibratory motion of the fan blades and vibratory motion of the surrounding fluid, it is in part an acoustical phenomenon, and the propensity of fan blades to flutter can be changed by manipulating the acoustical frequency response of the nacelle structures. Since the frequency at which fan blades flutter is different from the frequency of fan noise, efforts intended to modify the acoustical liner to improve fan blade flutter margin may be in conflict with the frequency response of the acoustical liner needed to reduce fan blade noise, thereby potentially leading to an increase in the fan blade noise. Discontinuities in acoustical frequency response between regions of the nacelle structure tuned to different frequencies for noise control and fan blade flutter control, respectively, can cause significant increases in noise. For example, acoustical resonators tuned to the fan blade flutter mode frequencies may be installed forward of the fan in an effort to reduce fan blade flutter. However, these acoustical resonators may conflict with the tuning of the inlet in regard to fan blade noise and result in a disadvantageous increase in the fan noise.
Accordingly, it would be desirable to provide for a mechanism for reducing or eliminating fan blade flutter without meaningfully increasing the weight of the engine, impairing the structural integrity of the engine and/or nacelle structures, or causing an increase in fan blade noise.